6 hours would be obviously stupid (seeing that if you play 2 hours each day, at the same time, you'd always see the same in-game time passing.

What they need to do is make day 7 hours or 5 hours, not 6. That way, each day you'd join in a different server time if you join the same time.

As this would create a cycle of day/night that is not stuck to the real world day night cycle this is the only solution. You simply have to desync the game time from real time. And only way to do that, is 11, 7, 5, 3 hours per day.